In telephone systems, line circuits serve as an interface between communication lines connected to subscriber sets and the telephone switching network. Traditionally, the line circuit has included a battery feed to energize the subscriber set. It has been recognized that the effect of longitudinal currents, which are frequently induced in a subscriber loop because of nearby electrical power lines, can be minimized when the battery feed is capable of floating with respect to ground. Known line circuits have implemented a floating battery feed by using a transformer to isolate the subscriber set from the battery. In such circuits, a second transformer is typically used to provide audio coupling between the subscriber set and the switching network. Because these transformers represent a significant fraction of the total cost and space requirements of the line circuits in which they are included, a line circuit which efficiently utilizes a single transformer both to pass energizing current to the subscriber set and to provide audio coupling between the switching network and the subscriber set has an obvious advantage over corresponding two-transformer circuits.
In accordance with one known line circuit, a transformer is used to energize a subscriber set by supplying a voltage at the circuit output terminals which is substantially independent of the length or DC resistance of the subscriber loop. Audio signals are passed between the switching network and the subscriber set using the same transformer. Although the advantage inherent in using a single transformer is achieved, constant voltage circuits supply currents to short, low-resistance loops far in excess of the current required to energize the subscriber sets and their use is, therefore, not energy efficient. A second, known single-transformer circuit provides a substantially constant current to the subscriber set independent of loop length. However, conventional subscriber sets are equipped with varistors, which advantageously act to limit the audio signals transmitted from and received by sets connected to short, low-attenuation loops. If constant current is supplied to such conventional sets, this desirable, loop-dependent varistor operation is no longer possible.
It has been determined that a battery feed which supplies a substantially constant magnitude of power independent of loop length is significantly more energy efficient than a constant voltage circuit and still allows for the use of varistors to limit audio signals on short loops. However, the dynamic output impedance of a constant power circuit will typically vary with the DC resistance of the subscriber loop, which may make the circuit incompatible with PBXs or other electronic circuits that terminate the loop. The present invention is a floating, constant power, battery feed circuit advantageously utilizing a single transformer to perform both the battery feed and audio coupling functions and having a predetermined, loop-independent, dynamic output impedance.